where \(\delta _{n}^{0}\)  is the intrinsic birefringence, which is a material constant for the orientational birefringence of polymers.
Importantly, within the EC regime, the birefringence can be treated as intrinsic birefringence, because the orientation angle remains constant at \(\pi /2\) .
In such situations, the liquid polymer filament is assumed to be an axially uniform cylindrical column of constant radius \(R_\mathrm {mid}\) , with a free-slip boundary condition, which does not induce a radial shear flow [1]}, [2]}.
Thus, the principal stress difference in Eq. (REF ) is equal to the normal stress, i.e., the uniaxially extensional stress \(\sigma _{e}\) , and the stress-optic law within the EC regime can be written as follows:
\(\delta = d \delta _n = C d \sigma _{e}.\) 
